Energy Conversion Using Rankine Cycle System

ABSTRACT

A process for recovering waste heat in an organic Rankine cycle system which comprises passing a liquid phase working fluid through heat exchange in successive communication with two or more process streams which thus heat the working fluid, removing a vapor phase working fluid from the heat exchanger, passing the vapor phase working fluid to an expander wherein the waste heat is converted into mechanical energy, and passing the vapor phase working fluid from the expander to a condenser wherein the vapor phase working fluid is condensed into the liquid phase working fluid.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to energy conservation in thecooling or condensing of process streams. Waste heat from processstreams may be converted in organic Rankine cycle systems intomechanical energy to generate electric power.

2. Discussion of the Background Art

Rankine cycle systems are known to be a simple and reliable means toconvert heat energy into mechanical shaft power. Organic working fluidsare useful in place of water/steam when low-grade thermal energy isencountered. Water/steam systems operating with low-grade thermal energy(typically 275° C. and lower) will have associated high volumes and lowpressures. Thus, a steam Rankine cycle using low-pressure steam as theworking fluid results in a large-sized steam turbine with lowpower-generation efficiency. To keep system size small and efficiencyhigh, organic working fluids with boiling points near room temperatureare employed. Such fluids would have higher gas densities lending tohigher capacity and favorable transport and heat transfer propertieslending to higher efficiency as compared to water at low operatingtemperatures. In industrial settings there are more opportunities to useflammable working fluids such as toluene and pentane, particularly whenthe industrial setting has large quantities of flammables already onsite in processes or storage. However, the ideal organic working fluidshould be environmentally acceptable, non-flammable, of a low order oftoxicity, and operate at positive pressures. Such fluids are disclosedin U.S. Pat. No. 7,428,816 B2, incorporated herein by reference thereto.

Organic Rankine cycle (“ORC”) systems are often used to recover wasteheat from industrial processes. These systems are particularlyappropriate when the potential thermal output is variable and directload matching becomes difficult, confounding efficient operation of thecombined heat and power system. In such an instance, it is useful toconvert the waste heat to shaft power by using an organic Rankine cyclesystem. The shaft power can be used to operate pumps, for example, or itmay be used to generate electricity. By using this approach, the overallprocess efficiency is higher and fuel utilization is decreased. Airemissions from energy production can be decreased since a higherproportion of demand for electric power is provided by the waste heat.

SUMMARY OF THE INVENTION

A broad embodiment of the invention is apparatus for generating powerfrom two or more process streams having different temperatures in anorganic Rankine cycle system, comprising one or more lower-temperatureexchangers for exchanging heat between at least one lower-temperatureprocess stream and a liquid Rankine-cycle working fluid to obtain aheated working fluid; one or more higher-temperature exchangers forexchanging heat between at least one higher-temperature process streamand the heated working fluid to obtain a vaporized working fluid; anexpander driven by the vaporized working fluid to produce power to anoutput shaft and a reduced-pressure working fluid; a working-fluidcondenser for reducing the temperature of the reduced-pressure workingfluid to obtain a liquid working fluid; a pump to circulate the liquidworking fluid in the cycle system; conduits connecting the one or morelower-temperature exchangers, higher-temperature exchangers, expander,condenser, pump, and working-stream bypasses around one or both of theexchangers; and, a controller for monitoring flow rates, temperaturesand pressures of the two or more process streams and working fluid andfor providing control signals to the pump and expander.

A more specific embodiment is an apparatus for generating power from twoor more process streams having different temperatures in an organicRankine cycle system, comprising: a lower-temperature condenser forcondensing at least one lower-temperature process stream by heating aliquid Rankine-cycle working fluid to obtain a heated working fluid; ahigher-temperature condenser for condensing at least onehigher-temperature process stream by heating the heated working fluid toobtain a vaporized working fluid; an expander driven by the vaporizedworking fluid to produce power to an output shaft and a reduced-pressureworking fluid; a working-fluid condenser for reducing the temperature ofthe reduced-pressure working fluid to obtain a liquid working fluid; apump to circulate the liquid working fluid in the cycle system; conduitsconnecting the lower-temperature condenser, higher-temperaturecondenser, expander, condenser, pump, and working-stream bypasses aroundone or both of the condenser; and, a controller for monitoring flowrates, temperatures and pressures of the two or more process streams andworking fluid and for providing control signals to the pump andexpander.

An alternative embodiment is a process for generating power from twoprocess streams having different temperatures in an organic Rankinecycle system, comprising cooling a lower-temperature process stream byheating a liquid Rankine-cycle working fluid to obtain a heated workingfluid; cooling a higher-temperature process stream by heating the heatedworking fluid to obtain a vaporized working fluid; expanding thevaporized working fluid to produce power to an output shaft and areduced-pressure working fluid; and, condensing the reduced-pressureworking fluid to obtain the liquid Rankine-cycle working fluid.

Additional objects, embodiments and details of this invention can beobtained and inferred from the following detailed description of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE is a simplified process flow diagram showing heat recoveryfrom two distillation columns using ORC.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

To better understand how organic Rankine cycle equipment can beconfigured to utilize waste heat from distillation columns, a diagram ofa basic equipment configuration is provided in the FIGURE.High-temperature column 10 effects separation of two or more componentsfrom feed 11 by distillation, employing reboiler 12 to provide heat tothe column. Overhead vapor in conduit 13 passes through section 100B ofRankine cycle exchanger 100 where it is condensed at least partially toliquid and routed via conduit 14 to receiver 15. In parallel,low-temperature column 20 effects separation of two or more componentsfrom feed 21 by distillation, employing reboiler 22 to provide heat tothe column. Overhead vapor in conduit 23 passes through section 100A ofRankine cycle exchanger 100 where it is condensed at least partially toliquid and routed via conduit 24 to receiver 25. The low-temperaturecolumn overhead transfers heat to the Rankine fluid entering theexchanger via conduit 101 before the high-temperature column overheadtransfers heat to the Rankine fluid, as this staged transfer is moreeffective in increasing the temperature of the heated Rankine fluid 102.The temperature difference between the two process streams leaving theexchanger, 24 and 14, preferably is at least about 10° C. although thisis not intended to limit the invention thereby.

The Rankine cycle system embodied in box 50 is not limited to condensingvapor streams such as 13 and 23. Any process streams exchanging heatwith Rankine-cycle working fluid in two or more sections of exchanger100, such as mixed-phase or liquid streams being cooled, are within theinvention. The invention is most effective in condensing servicesbecause the temperature ranges are smaller between the inlet and outletof process streams. Individual exchangers may be applied to the two ormore services shown in exchanger 100 rather than sections of the sameexchanger, i.e., exchangers 100A and 100B may be two separate exchangersin series in the ORC circuit.

The Rankine cycle system working fluid circulates through theheat-recovery heat exchangers 100 wherein it increases in temperatureand converts to vapor. The working fluid vapor is routed via conduit 102to the expander 103 where the expansion process results in conversion ofthe heat energy into mechanical shaft power. The shaft power can be usedto do any mechanical work by employing conventional arrangements ofbelts, pulleys, gears, transmissions or similar devices depending on thedesired speed and torque required. Importantly, the shaft can beconnected to an electric power-generating device such as an inductiongenerator. The electricity produced can be used locally or delivered tothe grid. Working fluid that exits the expander continues to thecondenser 104 where adequate heat rejection using either water (asshown) or air as a cooling medium causes the fluid to condense toliquid. It is also desirable to have a liquid surge tank 105 locatedbetween the condenser and pump to ensure there is always and adequatesupply of liquid to the pump suction. The liquid flows to a pump 106that elevates the pressure of the fluid to that it can be introducedback into the heat recovery heat exchanger thus completing the Rankinecycle loop.

When the Rankine cycle expander 103 is off-line or during transientconditions, such as start-up and shut-down, heat could be rejected toair or to water in condenser 104 as the working fluid continues tocirculate. The energy that would have been recovered in the expanderwould instead be rejected to the working fluid condenser 104. The heatexchanger designs can be fin/plate, plate/plate, shell/tube, fin/tube,microchannel, including double-wall or other designs that would beobvious to those skilled in the art.

Energy recovery according to the present invention, often involvingclose temperature approaches between process fluids, is improved throughthe use of exchangers having enhanced nucleate boiling surface. Suchenhanced boiling surface can be effected in a variety of ways asdescribed, for example, in U.S. Pat. No. 3,384,154; U.S. Pat. No.3,821,018; U.S. Pat. No. 4,064,914; U.S. Pat. No. 4,060,125; U.S. Pat.No. 3,906,604; U.S. Pat. No. 4,216,826; U.S. Pat. No. 3,454,081; U.S.Pat. No. 4,769,511 and U.S. Pat. No. 5,091,075; all of which areincorporated herein by reference. Such enhanced tubing is particularlysuitable for the exchange of heat to column reboilers and for rejectingheat from column condensers to other reboilers or to steam generators.

Typically, these enhanced nucleate boiling surfaces are incorporated onthe tubes of a shell-and-tube type heat exchanger. These enhanced tubesare made in a variety of different ways which are well known to thoseskilled in the art. For example, such tubes may comprise annular orspiral cavities extending along the tube surface made by mechanicalworking of the tube. Alternately, fins may be provided on the surface.In addition the tubes may be scored to provide ribs, grooves, a porouslayer and the like.

Generally, the more efficient enhanced tubes are those having a porouslayer on the boiling side of the tube. The porous layer can be providedin a number of different ways well known to those skilled in the art.The most efficient of these porous surfaces have what are termedreentrant cavities that trap vapors in cavities of the layer throughrestricted cavity openings. In one such method, as described in U.S.Pat. No. 4,064,914, the porous boiling layer is bonded to one side of athermically conductive wall. An essential characteristic of the poroussurface layer is the interconnected pores of capillary size, some ofwhich communicate with the outer surface. Liquid to be boiled enters thesubsurface cavities through the outer pores and subsurfaceinterconnecting pores, and is heated by the metal forming the walls ofthe cavities. At least part of the liquid is vaporized within the cavityand resulting bubbles grow against the cavity walls. A part thereofeventually emerges from the cavity through the outer pores and thenrises through the liquid film over the porous layer for disengagementinto the gas space over the liquid film. Additional liquid flows intothe cavity from the interconnecting pores and the mechanism iscontinuously repeated. Such enhanced tubes containing a porous boilinglayer are commercially available under the trade name High Flux Tubingmade by UOP, Des Plaines, Ill. To achieve minimum temperaturedifferences across exchanger 100 and thus improve the efficiency of theRankine cycle, enhanced nucleate boiling surfaces are preferred withinthe present invention for vaporizing the working fluid in exchanger 100.

Enhanced condensing surfaces are also useful for practical heatexchanger designs with small temperature approaches. Enhanced condensingsurfaces are preferred within the present invention for either theoverhead condensers in 100 or for the working fluid condenser in 105.

The apparatus comprises a controller for monitoring flow rates,temperatures and pressures of the two or more process streams andworking fluid and for providing control signals to the pump andexpander. An electronic controller as known to the skilled routineer islinked to the numerous components of an energy-conversion system tomonitor and/or control the operation of the component typically based onset points or operating points in memory or electronically set withinthe controller. The electronic controller is linked inter alia to theexpander, condenser, pump, and working-stream bypasses around one orboth of the condensers. Of course, the electronic controller can beimplemented using multiple controllers with the important concept beingmaintenance relatively steady operations even in periods of varyinginput fluid temperatures or the variance of other operating parameters.

Organic compounds often have an upper temperature limit above whichthermal decomposition will occur. The onset of thermal decompositionrelates to the particular structure of the chemical and thus varies fordifferent compounds. In order to access a high-temperature source usingdirect heat exchange with the working fluid, design considerations forheat flux and mass flow, as mentioned above, can be employed tofacilitate heat exchange while maintaining the working fluid below itsthermal decomposition onset temperature. Direct heat exchange in such asituation typically requires additional engineering and mechanicalfeatures which drive up cost. In such situations, a secondary loopdesign may facilitate access to the high-temperature heat source bymanaging temperatures while circumventing the concerns enumerated forthe direct heat exchange case. This approach also can provide morefreedom to retrofit to future improved working fluids in the Rankinecycle system without having to disturb or alter the process heatrejection package. A cost-risk/benefit analysis is often conducted inorder to determine the best approach (direct or indirect heat exchange)for a particular application.

Fluid selection depends on a variety of factors including temperaturematch, thermodynamic properties, heat transfer properties, cost, safetyconcerns, environmental acceptability, and availability. Working fluidsthat are suitable include water, silicones, aliphatic hydrocarbons,cyclic hydrocarbons, aromatic hydrocarbons, olefins, hydrofluorocarbons(including alkanes and alkenes, cyclic compounds), hydrofluoroethers,perfluoroethers, alcohols, ketones, fluorinated ketones, fluorinatedalcohols, esters, phosphate esters. Other fluids that are suitable aredescribed in U.S. Pat. No. 7,428,816 B2 which is incorporated herein byreference. In addition to the fluids mentioned above for use in theprocesses of this invention, a number of preferred fluids have beenidentified that are useful in the processes of this invention. Includedamong the fluids that are useful in the process of the invention are thepreferred compounds of the structure.

where x, y, z, and m are each selected from the group consisting of:fluorine, hydrogen, R_(f), and R, wherein R and R_(f) are each an alkyl,aryl, or alkylaryl of 1 to 6 carbon atoms, and wherein R_(f) ispartially or fully fluorinated. Also among the preferred are saturatedcompounds derived by reacting the aforementioned compounds with HF andthose compounds derived by reduction with hydrogen.

Most preferred are compounds of the formula CxFyHz where x=12−b where bis from 0 to 6, y=2x−z, and for x/2 and 2x/3=integers then z=2x/3; forx/2 and 3x/4=integers then z=3x/4; for x/2.noteq.integer then z=x−2; forx/2 and x/5=integers then z=x−3. Also most preferred are the saturatedcompounds derived by reacting HF with the aforementioned compounds andthose compounds derived by reduction with hydrogen. Genetron 245fa(HFC-245fa) is a particularly favored working fluid.

Example

The following example illustrates the benefits of the invention usingORC in an aromatics complex producing 900,000 tons/year of para-xylene.Aspects of the aromatics complex are described in U.S. Pat. No.6,740,788 which is incorporated herein by reference. Columns 10 and 20as described in the FIGURE of the present application are, respectively,distillation columns in an adsorption separation unit separatingC₈-aromatics raffinate from desorbent and para-xylene-rich extract fromdesorbent. The raffinate column is, relatively, the high-temperaturecolumn and the extract column is the low-temperature column. To compareORC to air cooling and water cooling, the heat duties are as follows:

Heat Rejected (MW) T-in (° C.) T-out (° C.) Extract Column Overhead 34.0151.1 128.9 Raffinate Column Overhead 90.6 152.6 140.8 Total 124.6 — —

The net power benefit of using ORC relative to air and water condensercases is calculated using the following equations:

For Water Cooling:

Net Power Benefit (MW)=Turbine Power Generation (MW)−Pump Power(MW)−Cooling Water Pump Power (MW)−Cooling Tower Fan Power (MW)+BaseCase Air Cooler Fan Power (MW)

For Air Cooling:

Net Power Benefit (MW)=Turbine Power Generation (MW)−Pump Power (MW)−AirCooler Fan Power (MW)+Base Case Air Cooler Fan Power (MW)

Water Condenser Air Condenser Net power benefit (MW) 13.2 12.4 AnnualBenefits ($MM/year) Cases Water Condenser Air Condenser Power =$0.07/kWh  $7.4 MM $6.9 MM No CO₂ credits Power = $0.10/kWh $10.6 MM$9.9 MM No CO₂ credits Power = $0.07/kWh  $9.1 MM $8.4 MM 30$/MT CO₂credits Power = $0.10/kWh $12.3 MM $11.3 MM  $30/MT CO₂ credits

Assumptions for Power Generation Calculations Turbine/generatorefficiency 0.80/0.95 Cooling tower fans 7 kW/1000 gpm Cooling water pumpΔP 50 psi Fuel equivalent of power 9,090 Btu/kWh CO₂ emissions 56.2kg/GJ

1. An apparatus for generating power from two or more process streamshaving different temperatures in an organic Rankine cycle system,comprising: (a) one or more lower-temperature exchangers for exchangingheat between at least one lower-temperature process stream and a liquidRankine-cycle working fluid to obtain a heated working fluid; (b) one ormore higher-temperature exchangers for exchanging heat between at leastone higher-temperature process stream and the heated working fluid toobtain a vaporized working fluid; (c) an expander driven by thevaporized working fluid to produce power to an output shaft and areduced-pressure working fluid; (d) a working-fluid condenser forreducing the temperature of the reduced-pressure working fluid to obtaina liquid working fluid; (e) a pump to circulate the liquid working fluidin the cycle system; (f) conduits connecting the one or morelower-temperature exchangers, higher-temperature exchangers, expander,condenser, pump, and working-stream bypasses around one or both of theexchangers; and, (g) a controller for monitoring flow rates,temperatures and pressures of the two or more process streams andworking fluid and for providing control signals to the pump andexpander.
 2. The apparatus of claim 1 further comprising a generator toproduce electric power connected to the output shaft of the expander. 3.The apparatus of claim 1 wherein one or more of the lower-temperatureand higher-temperature exchangers are condensers for condensing at leastpart of the respective process streams from vapor phase into liquidphase.
 4. The apparatus of claim 3 wherein one or more of the condensersare overhead condensers of distillation columns.
 5. The apparatus ofclaim 1 wherein one or more of the lower-temperature andhigher-temperature exchangers is a reactor effluent cooler.
 6. Theapparatus of claim 1 wherein at least one of the lower-temperatureexchangers is a product cooler.
 7. The apparatus of claim 1 wherein oneor more of the exchangers has an enhanced nucleate boiling surface. 8.The apparatus of claim 1 wherein one or more of the condensers has anenhanced condensing surface.
 9. The apparatus of claim 1 wherein theworking fluid comprises R-245a.
 10. The apparatus of claim 1 comprisingtwo or more low-temperature exchangers, each for exchanging heat betweenat least one lower-temperature process stream and a liquid Rankine-cycleworking fluid to obtain a heated working fluid;
 11. An apparatus forgenerating power from two or more process streams having differenttemperatures in an organic Rankine cycle system, comprising: (a) alower-temperature condenser for condensing at least onelower-temperature process stream by heating a liquid Rankine-cycleworking fluid to obtain a heated working fluid; (b) a higher-temperaturecondenser for condensing at least one higher-temperature process streamby heating the heated working fluid to obtain a vaporized working fluid;(c) an expander driven by the vaporized working fluid to produce powerto an output shaft and a reduced-pressure working fluid; (d) aworking-fluid condenser for reducing the temperature of thereduced-pressure working fluid to obtain a liquid working fluid; (e) apump to circulate the liquid working fluid in the cycle system; (f)conduits connecting the lower-temperature condenser, higher-temperaturecondenser, expander, condenser, pump, and working-stream bypasses aroundone or both of the condenser; and, (g) a controller for monitoring flowrates, temperatures and pressures of the two or more process streams andworking fluid and for providing control signals to the pump andexpander.
 12. The apparatus of claim 11 wherein one or more of thelower-temperature and higher-temperature exchangers are condensers forcondensing at least part of the respective process streams from vaporphase into liquid phase.
 13. The apparatus of claim 12 wherein one ormore of the condensers are overhead condensers of distillation columns.14. The apparatus of claim 11 further comprising a generator to produceelectric power connected to the output shaft of the expander.
 15. Theapparatus of claim 11 wherein one or more of the exchangers has anenhanced nucleate boiling surface.
 16. The apparatus of claim 11 whereinthe working fluid comprises R-245a.
 17. A process for generating powerfrom two process streams having different temperatures in an organicRankine cycle system, comprising: (a) cooling a lower-temperatureprocess stream by heating a liquid Rankine-cycle working fluid to obtaina heated working fluid; (b) cooling a higher-temperature process streamby heating the heated working fluid to obtain a vaporized working fluid;(c) expanding the vaporized working fluid to produce power to an outputshaft and a reduced-pressure working fluid; and, (d) condensing thereduced-pressure working fluid to obtain the liquid Rankine-cycleworking fluid.
 18. The process of claim 17 further comprising generatingpower via a generator connected to the output shaft of the expander. 19.The process of claim 17 wherein one or more of the process streamscomprise overhead vapors of distillation columns and are condensed atleast partially to liquid streams.
 20. The process of claim 17 whereinthe working fluid comprises R-245a.